The inkjet recording method is a recording method in which ink liquid droplets are discharged from extremely fine nozzles and adhered directly to a recording member to obtain text or an image. This method not only has the advantages that the apparatus used generates little noise and is easy to operate, but also offers the advantages that creating colors is easy, and plain paper can be used as the recording member. Accordingly, these types of apparatus are widely used as output devices in offices and households.
On the other hand, even in industrial applications, improvements in inkjet technology mean the use of inkjet devices as output devices for digital printing is now expected, and printing devices using solvent-based inks and UV inks that are capable of printing even to low-absorption print media (including plastic substrates such as PVC and PET) are actually commercially available. However, in recent years, because of the need to satisfy environmental requirements, the demand for aqueous inks is increasing.
As described in Patent Documents 1, 2 and 3, inks for printing to plain paper and specialty papers such as photographic glossy paper have long been available as aqueous inks suitable for inkjet printing. However, in recent years, expectations have grown for an expansion in the potential applications for inkjet recording systems, and there are growing needs for direct printing to coated papers and low-absorption printing substrates such as those used in industrial applications. With conventional inks, liquid droplets are absorbed into the paper to create an image, and therefore if printing is performed onto a printing substrate having low water absorption, the image tends to blur, meaning such conventional inks cannot be used.
In industrial applications, printing to transparent substrates such as PET films and printing to colored substrates are becoming increasingly necessary. If only process color inks are printed to these substrates, then the color effect of the substrate means that an image having inferior coloration and visibility is sometimes obtained. In order to address this issue, methods that also use white inks are known. In other words, by subjecting the substrate to an undercoating treatment by printing a white ink prior to printing the color inks, and subsequently forming an image with the color inks on top of this undercoat, an image that exhibits excellent color development of the color inks and superior visibility can be obtained. Further, in a similar manner, by subjecting the substrate to an undercoating treatment by printing a color ink prior to printing the white ink, and subsequently forming an image with the white ink on top of this undercoat, an image that exhibits excellent color development of the white ink and superior visibility can be obtained. Accordingly, developments aimed at practical applications of white aqueous inkjet inks continue to be pursued (Patent Documents 4 and 5).
However, if a white ink such as those disclosed in the aforementioned documents is printed onto a low-absorption substrate such as a PET film to undercoat the substrate, and a color ink is then printed on top of that undercoat, a problem arises in that mixed color bleeding occurs between the white ink and the color ink, making it difficult to obtain a sharp image. Further, in a similar manner, if a color ink is printed first to undercoat the substrate, and a white ink is then printed on top of that undercoat, mixed color bleeding occurs, and superior image quality cannot be obtained.